This invention relates generally to a blender for mixing various materials with a fluid, according to a predefined ratio. More particularly, the invention relates to a blender, for use in the oil field industry, which mixes sand and chemicals with water during a well fracturing operation.
During a well fracturing operation, a high volume of water is pumped into a wellbore, and thus into the oil or gas producing formation, in order to create fissures in the formation. The fissures that are created during the well fracturing operation provide a passage through which the oil or gas may more readily flow. The water is pumped into the formation at a rate that exceeds the absorption capacity of the formation, and, as a result, cracks, or fissures, form in the formation.
After the fissures have been formed, sand is mixed with the water in a blender according to a predefined ratio, thereby increasing the density of the water. The sand is added to fill the fissures that have been created full of sand. In this manner, the fissures are held open even when the water is no longer pumped into the formation. Because the sand is porous, oil and gas may still flow through the fissures. The fluid output from the blender that is pumped into the wellbore is commonly called the "slurry."
As shown in FIG. 1, the sand is added in an increasing proportion with the water in order to provide a stepped increase in the water density. Thus, for example, after 100,000 gallons of water have been pumped into the formation, sand is mixed with the water to raise the water density to 1.10 specific gravity units ("SGU's"). After another fifty thousand gallons of the 1.10 SGU slurry has been pumped into the formation, an increased amount of sand is mixed with the water to raise the density of the slurry to 1.20 SGU. This stepped increase may continue until the operation is completed. In this manner, the amount of sand is increased based on the cumulative addition of water into the formation. This is done because the lower the amount of sand in the water (the lower the density), the further that the sand will travel in the formation. Increasing the amount of sand in this stepped fashion ensures that the outermost point of the fissures will be filled before the inner points are filled. As a result, the sand is better distributed throughout the fissured formation.
As the density of the slurry increases, and as the fissures begin to fill with sand, the pressure necessary to pump the slurry into the wellbore increases. The fracturing process is terminated when the pressure reaches a preselected level.
A plurality of chemicals may also added to the sand/water mixture in the blender according to a predefined ratio.
A number of problems arise in the prior art fracturing process. The primary problem is the difficulty in continuously maintaining the proper ratio of the sand to water and of the chemicals to water. Because the addition of sand and chemicals to the water is manually controlled, a great deal of fluctuation occurs in both the density level of the water and the proportion of the chemicals to the water. The elimination of human error would greatly increase the efficiency of the well fracturing operation. In addition, manual control of the well fracturing operation necessitates the addition of the sand in definitive steps. The fracturing process would be more efficient if the amount of sand was continuously increased as shown in FIG. 2. See Thomas M. Hopkins, "Technique Helps Extend Cotton Valley Frac", Petroleum Engineer International, January 1989. In such a process the sand would be continuously increased to slow the pressure increase and to provide for a better distribution of sand during the fracturing process. This type of continuous increase in slurry density is often called a ramp increase. It is virtually impossible for an operator to manually increase the proportion of sand in the water in accordance with an idealistic ramp schedule.
Yet another problem with the prior art fracturing process is that there is little or no calibration or feedback adjustment based on the actual density of the water being pumped into the well. Failure to monitor the slurry pumped into the wellbore can result in gross errors if there is a malfunction in the equipment used to deliver the sand or chemicals.